Means and method for controlling the hydrocarbon content of recycle acid in an alkylation unit

ABSTRACT

A control system and method controls the hydrocarbon of recycle acid in an alkylation unit to maintain a predetermined hydrocarbon content. The hydrocarbon content is controlled by controlling the interface level between the acid phase and the hydrocarbon phase in an acid settler in accordance with sensed densities of fresh acid entering the alkylation unit, crude alkylate provided by the acid settler in the alkylation unit and the recycle acid. A network substantially determines the actual hydrocarbon content of hydrocarbon enriched acid leaving the acid settler a portion of which is provided as the recycle acid, along with the fresh acid, to a contactor. In the contactor, an isoparaffin and an olefin are contacted with the acid to provide an acid-hydrocarbon mix to the acid settler. The settler provides the crude alkylate for further processing.

United States Patent [191 Sweeney, Jr. et al.

[ MEANS AND METHOD FOR CONTROLLING THE HYDROCARBON CONTENT OF RECYCLEACID IN AN ALKYLATION UNIT [75] Inventors: Donald E. Sweeney. Jr.,Beaumont;

Walker L. Hopkins, Houston. both of Tex.

[73] Assignee: Texaco Inc., New York, NY.

[22] Filed: Aug. 16, 1972 211 Appl. No.: 281,063

[52] U.S. Cl...... 235/15l.l2, 23/255 E, 208/DIG. 1, 260/683.58

[51] Int. Cl. C07c 3/52, G06g 7/58 [58] Field of Search..... 260/683.58,683.48, 683.59, 260/683.62, 683.43; 235/l51.12, 150.1;

208/133, 134, DIG. 1; 23/230 R, 232 E, 252

R, 253 R, 254 E, 255 E DENSITY ANALYZER CONTACTOR ISOPARAFIN 8 OLEFINSDISCHARGE ACID DENSITY ANALYZER June 25, 1974 Hopkins et al 260/683.43Child et al. 260/68359 [5 7] ABSTRACT A control system and methodcontrols the hydrocarbon of recycle acid in an alkylation unit tomaintain a predetermined hydrocarbon content. The hydrocarbon content iscontrolled by controlling the interface level-between the acid phase andthe hydrocarbon phase in an acid settler in accordance with senseddensities of fresh acid entering the alkylation unit, crude alkylateprovided by the acid settler in the alkylation unit and the recycleacid. A network substantially determines the actual hydrocarbon contentof hydrocarbon enriched acid leaving the acid settler a portion of whichis provided as the recycle acid, along with the fresh acid, to acontactor. In the contactor, an isoparaffin and an olefin are contactedwith the acid to provide an acid-hydrocarbon mix to the acid settler.The settler provides the crude alkylate for further processing.

9 Claims, 2 Drawing Figures CRUDE ALKYLATE INTERFACE LEVEL HYROCARBONPHASE ACID PHASE DENSITY ANALYZER ACID SETTLER PATENTEnJuazs 1974 saw 1or 2' xATENIED 1W4 SHEET 2 (IF 2 1 MEANS AND METHOD FOR CONTROLLING THEHYDROCARBON CONTENT OF RECYCLE ACID IN AN ALKYLATION UNIT BACKGROUND OFTHE INVENTION 1. Field of the Invention The system of the presentinvention relates to control systems and methods in general and, moreparticularly, to a control system and method for an alkylation unit.

2. Description of the Prior Art Heretofore, level control systems foracid settlers controlled the interface level in an acid settler to apredetermined value. As the composition of the olefin and isoparaffinfeed changed or the temperature changed, the hydrocarbon content ofrecycle acid from the settler changed accordingly and differed from thatnormally provided by the predetermined interface level.

The present invention controls the interface level in the acid settleras a function of the hydrocarbon 'content of the hydrocarbon enrichedacid leaving the settler. In doing so, the system senses the densitiesof the hydrocarbon enriched acid, the crude'alkylate and the fresh acidwhich are used to determine the hydrocarbon content of the hydrocarbonenriched acid.

SUMMARY OF THE INVENTION A system controls the interface level betweenthe acid phase and the hydrocarbon phase in an acid settler in analkylation unit. The settler receives an acidhydrocarbon mix from acontactor which has reacted an olefin and an isoparaffin in the presenceof an acid catalyst. The settler provides crude alkylate and hydrocarbonenriched acid a portion of which is recycled to the contactor. A portionof the hydrocarbon enriched acid from the settler is discharged andreplaced by fresh acid entering the alkylation unit. The control systemincludes sensors sensing conditions of the fresh acid. the crudealkylate and the recycle acid. A network provides a signalcorrespondingto the hydrocarbon content of the recycle acid in accordance with thesensed conditions. A control signal is developed in accordance with thehydrocarbon content signal and a signal correspondng to a predeterminedhydrocarbon content. The control signal is applied to control meanswhich control the interface level in the settler so as to controlthehydrocarbon content of the recycle acid.

The objects and advantages of the invention will appear more fullyhereinafter from a consideration of the detailed description whichfollows, taken together with the accompanying drawings wherein oneembodiment of the invention is illustrated by way of example. It is tobe expressly understood, however, that the drawings are for illustrationpurposes only and are not to be construed as defining the limits of theinvention.

DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE INVENTION Referring toFIG. 1, there is shown a portion of an alkylation unit in which anolefin is reacted with an iso- 5 parafi'm in the presence of a catalyst,such as sulphuric or hydrofloric acid, and which is hereinafter referredto as acid, to fonn a higher molecular weight isoparaffin. For purposeof explanation, the acid in the following description shall be sulphuricacid. The olefin may be butylenes, propylene, or a mixture of butylenesand propylene, while the isoparaffin may be isobutane.

The olefins and isoparaffin enter a contactor 4, by way of a line 6,where the olefin and isoparaffin are contacted with recycle acidentering byway of a line 7. Contactor 4 provides an acid-hydrocarbon mixby way of a line 8 to an acid settler l2. Settler l2 separates crudealkylate from the acid and the hydrocarbon product is provided through aline 14 while the acid is removed by way of a line 16. It should benoted that the acid from settler 12 is an acid which is enriched withsome hydrocarbon that has not been thoroughly separatedby the action ofsettler l2. Acid settler 12 may be the only acid settler in the unit orit may be any acid settler in a group of acid settlers. Inside acidsettler 12, a hydrocarbon phase is separated from an acid phase by aninterface level. Fresh acid enters line 16 by way of a line 17 as neededto maintain a desired acid strength. A pump 20 pumps the acid from line16 into line 7. A portion of the acid in line 7 is discharged by way ofa line 21 leaving the recycle acid to be provided to contactor 4. Thedischarged acid may be provided to another alkylation unit or disposedof.

The crude alkylate in line 14 can have its octane rating increased byincreasing the amount of hydrocarbon in the acid leaving settler 12.There are several variables that could affect the hydrocarbon in line16. One variable is settler l2 interface level between the acid phaseand the hydrocarbon phase. Another variable is the temperature withinsettler 12. The type and quantity of the olefins in line 6 will alsoaffect the hydrocarbon content in the recycle acid in line 16.

In practice it may not be feasible to maintain the various variables atconstant values. Hence, the control of the interface level in settler 12at a constant value will not necessarily maintain the hydrocarboncontent in the recycle acid in line I6 at a constant value. It istherefore desirable to control the interface level in acid settler 12 asa function of the hydrocarbon content of the recycle acid in line 16.

The hydrocarbon content of the recycle acid in line 16 can be related tothe density of the recycle acid in line 16, the density of the acidportion of the recycle acid and the density of the hdyrocarbon portionof the recycle acid by the following equation 1.

H, D, Din/(D4 D) bining X volume units of acid with Y volume units of ihydrocarbon will result in (X Y) volume units of mixture. Although thismay not be exactly true in practice, the error introduced by thisapproximation does not materially affect the application of theinvention.

Since fluid density is a function of temperature, it is necessary thatall density values be adjusted to a common temperature base, such as 60F, before determining existing recycle acid hydrocarbon content fromequation 1.

The interface level in settler 12 is also affected by a valve 25 in line21 cooperating with a flow rate sensor 26 and a flow recorder controller27. The set point of flow recorder controller 27 may be preset for apredetermined flow rate. Sensor 26 provides a signal to controller 27corresponding to the actual flow rate in line 21. Flow rate recordercontroller 27 then provides a signal to valve 25 to control the flowrate so that the flow rate in line 21 assumes the flow rate as indicatedby the position of the set point of controller 27. The discharge acidflow rate is maintained at a value so that the interface level may becontrolled by controlling the fresh acid flow rate as hereinafterexplained.

Density analyzers 30, 31 and 33 provide signals E E and E respectively,corresponding to the density of the recycle acid in line 7, to freshacid in line 17 and the crude alkylate in line 14, respectively. Thefresh acid density is used as the density of the acid portion density Dof the recycle acid, while the crude alkylate density is used as thehydrocarbon portion density D Density analyzers 30, 31, 33 may by way ofexample be devices manufactured by Automation Products Incorporatedunder the name Dynatrol Density-Specific Gravity Control Device. Thedevices are adjustable to control the range of operation. For example,analyzer 31 might include a range andspan of 1.5 to 2.0 specific gravityunits at an operating temperature range of 45 to 70 F. While densityanalyzer 30 may have a specified range and span of I34 to 1.84 specificgravity units and density analyzer 33 may have a specified range andspan of 0.5 to L specific gravity units.

A conventional type level sensor 40 provides a signal E corresponding tothe interface level to a level recorder controller 41, which may be of aconventional type, and a set point signal means 45 which also receivessignals E,, E and E Set point signal means 45 provides a control signalE to level recorder controller 41 to adjust its set point, accordingly.In accordance with signals E through E,,, equation 1 and otherlimitations herinafter described the set point is changed accordingly.Level recorder controller 41 controls the interface level in settler 12as follows: When the interface level in settler 12 is greater than thelevel determined by the position of the set point in controller 41,controller 41 provides a signal to a valve 49 in line 17 reducing theflow rate of the fresh acid in line 17. Since the discharge acid flowrate is at a constant rate, the acid provided to contactor 4 is reducedand hence the acid to settler 12 is thereby reduced until such time asthe interface level in settler 12 is substantially equal to the leveldetermined by the position of the set point in controller 41.

When the interface level in settler 12 is less than the level asdetermined by the position of the set point in controller 4], controller41 provides a signal to valve 49 increasing the flow rate of the freshacid in line 17 thereby increasing the acid entering contactor 4 andacid settler 12 which results in the interface level in settler 12increasing until such time it is substantially equal to the level asdetermined by the position of the set point in controller 41.

Referring to FIG. 2, set point signal means 45 is placed in operation bythe closure of a conventional type on-off toggle switch 55. When closed,switch 55 passes a direct current voltage V, from a source 56 of directcurrent voltages to clock means 58 thereby energizing clock means 58causing it to provide sampling pulses E on a periodic basis. Source 56also provides direct current voltages V through V Sampling pulses E, areapplied to sample and hold circuits 60, 60A and 60B causing them tosample and hold signals E through E respectively. Elements having asuffix are connected and operate in a similar manner as elements havingthe same number without a suffix. Subtracting means 63 subtracts theoutput from sample and hold circuit 60 from the output from sample andhold circuit 60A to provide a signal corresponding to the term (D,,--D,,) in equation 1. Similarly, subtracting means 63A subtracts theoutput provided by sample and hold circuit 608 from the output providedfrom sample and hold circuit 60A to provide a signal corresponding tothe term (D D A divider 65 divides the signal from subtracting means 63with the signal from subtracting means 63A to provide an output which ismultiplied with voltage V by a multiplier 68. Voltage V corresponds tothe term 100 in equation 1. Multiplier 68 provides a signal Ecorresponding to the determined hydrocarbon content H,, of the recycleacid in line 16 and 7.

A required change AL in the interface level so that the hydrocarboncontent of the recycle acid is substantially equal to a targethydrocarbon content is determined from equation 2:

AL @(Hg H1) where e is aconstant, and may have a value of 0.5, and H isthe target hydrocarbon content.

Voltage V;,, corresponding to a target hydrocarbon content of therecycle a'cid H is subtracted from signal E,, by subtracting means toprovide a signal corresponding to the term (H H in equation 2. Amultiplier 77 multiplies voltage V, with the signal from subtractingmeans 75 to provide a signal E corresponding to the term AL inequation,2.

The change in the interface level required to achieve the desired levelmay in itself have some limitations placed on it. In order to preventlarge and rapid changes in settler interface level and hydrocarboncontent of the recycle acid which could upset smooth operation of thealkylation unit, a limitation is placed on the amount which the settlerlevel can be changed at one time. In this regard, voltage V which ispositive, and voltage V which is negative, correspond to the largestincrease in settler level AL which will be permitted and to the largestdecrease in settler level AL which will be permitted, respectively.Comparator 80, 80A compare signal E with voltages V and V respectively.For the condition that the required change in the interface level doesnot equal or exceed a limit, comparators 80, 80A provide high leveldirect current outputs which cause an AND gate 81 to provide a highlevel direct current output to anelectronic switch 83. Switch 83 isrendered conductive by the high level output from AND gate 8 and passessignal E from multiplier 77 as a change signal E The high level outputsfrom comparators 80, 80A are inverted to low levels by inverters 85 and85A, respectively, to render electronic switches 83A and 833,respectively, nonconductive. While non-conductive switches 83A, 83Bblock voltages V and V respectively.

When the required change exceeds the upper change limit AL the upperchange limit is used as the change to the interface level. Thus, underthe last mentioned condition, comparator 80 provides a low level outputwhich is inverted to a high level by an inverter 85 to render switch 83Aconductive. When rendered conductive, electronic switch 83A passespositive voltage V, as change signal E Comparator 80A provides a highlevel output, which is inverted to a low level by inverter 85A therebyrendering switch 833 nonconductive to block voltage V,,. AND gate 81provides a low level output in response to the output from comparator 80causing switch 83 to block signal E When the required change is areduction in level equal to or greater than lower change limit AL thelower change limit is used as the change to the interface level. Underthe last mentioned condition, comparators 80, 80A provide a high leveloutput and a low level output, respectively. lnverters 85 and 85Aprovide a low level output and a high level output, respectively,thereby rendering switch 83A non-conductive and switch 838 conductive,respectively. Switch 838 when rendered conductive passes negativevoltage V as signal E AND gate 81 provides a low level output inresponse to the output from-comparator 80A causing switch 83 to blocksignal E,,,. The inverted output from comparator 80 causes switch 83A toblock voltage V A desired interface level is determined by algebraicallysumming change level signal E with the sensed interface level signal E,which is accomplished by summing means 90. Summing means 90 provides asignal E corresponding to the desired interface level to comparators 93,93A. Comparators 93, 93A also receive voltages V, and V,,, respectively,corresponding to an upper limit L,- and a lower limit L,,, respectively,for the interface level in settler 12. Voltage V, is more positive thanvoltage V,,. When the desired level E is less than the upper level limitand greater than the lower level limit, comparators 93, 93A provide highlevel direct current outputs to an AND gate 97 and to inverters 98, 98A.The high level outputs cause AND gate 97 to provide a high level directcurrent output to an electronic switch 100 rendering it conductive.Switch 100 when rendered conductive passes signal E as a new set pointsignal E Inverters 98, 98A invert the high level outputs fromcomparators 93, 93A, respectively, to a low level rendering electronicswitches 100A and 1008 non-conductive. When rendered non-conductiveswitches 100A and 100B block voltages V V,,, respectively.

For the condition that the desired interface level is equal to orgreater than the upper level limit, the upper limit is used as the newinterface level. Under the last mentioned condition. comparator 93 and93A provide a low level output and a high level output, respectively.The output from comparator 93 causes AND gate 97 to provide a low leveloutput to switch 100 rendering it non-conductive. Switch 100 whennon-conductive blocks signal E lnverter 98 inverts the low level outputfrom comparator 93 to a high level to render switch 100A conductivethereby providing voltage V, as signal E The high level output fromcomparator 93A is inverted to a low level by inverter 98A causing it torender switch B non-conductive to block voltage V,,. For the conditionthat the desired level isequal to or less than the lower limit level Lthe lower level limit is used as the new interface level. Under the lastmentioned condition, comparators 93, 93A provide a high level output anda low level output, respectively. AND gate 97, in response to the lowlevel output from comparator 93A, provides a low level output to switch100 rendering it non-conductive to block signal E The high level outputfrom comparator 93 is inverted to a low level to render switch 100Anon-conductive thereby blocking voltage V The low level output fromcomparator 93A is inverted to a high level by inverter 98A renderingswitch 100B conductive to pass voltage V3 as Signal E14.

Signal E is applied to a sample and hold circuit 110. Sample and holdcircuit is controlled so that it will sample and hold signal E at apredetermined time after the sampling and holding of signals E,, E andE, by circuits 60, 60A and 608, respectively, to allow for theaforementioned computations to be made. Clock means 58 provides a pulseE to a one-shot multivibrator 1 l2. Multivibrator l 12 acts as a timedelay, the time delay being the width of the pulse provided bymultivibrator 112. The trailing edge of the pulse from multivibrator 112is triggered to another one-shot multivibrator 114. Multivibrator 114when triggered provides a sampling pulse to sample and hold circuit 110causing it to sample and hold signal E Sample and hold circuit 110provides signal E to level recorder controller 41 to adjust the setpoint in controller 41.

The control of interface level in settler 12 has been heretoforedescribed by controlling the flow rate of the fresh acid. It would beobvious to one skilled in the art, that the interface level may also becontrolled by controlling the flow rate of the discharge acid whilemaintaining a constant flow rate for the fresh acid.

A digital computer may be used in lieu of set point signal means 45. Itis necessary that signals E, through E, be converted to digital signals,which may be done by a conventional type analog-to-digital converters.The digital signals are applied to a general purpose digital computer,which may be of a conventional type. The digital computer is programmedto solve equations 1, 2 and to carry out the selective functions withrespect to the interface level change and the interface level, (i.e.,AL, AL or AL and L, L or L The digital output from the digital computercorresponding to the desired interface level is converted by aconventional digital-to-analog converter. The digital-to-analogconverter provides signal E to flow recorder controller 41 to controlthe interface level in acid settler 12 as heretofore explained.

The system of the present invention as heretofore described maintainsthe hydrocarbon content of recycle acid in an alkylation unit, bycontrolling the interface level between the acid phase and hydrocarbonphase in an acid settler in the alkylation unit. The level in thesettler is controlled as a function of the determined hydrocarboncontent of the recycled acid. The hydrocarbon content of the recycledacid is determined from the density of fresh acid entering thealkylation unit of the recycle acid and of crude alkylate provided bythe settler.

What is claimed is:

l. A system for controlling an alkylation unit to maintain thehydrocarbon content of recycle acid in the alkylation unit and saidalkylation unit includes a contactor wherein an olefin-isoparafi'mmixture is contacted with acid and the contactor provides anacidhydrocarbon mixture to an acid settler which separates a hydrocarbonphase from an acid phase to provide hydrocarbon enriched acid andprovide crude alkylate, a portion of the hydrocarbon enriched acid isdischarged while a portion of the hydrocarbon enriched acid is recycledback to the contactor as the recycle acid along with fresh acid enteringthe alkylation unit, comprising means for sensing conditions of thefresh acid, the crude alkylate and the recycle acid and providing asignal corresponding to the hydrocarbon content of the recycle acid'inaccordance with the sensed conditions, means for providing a signalcorresponding to a predetermined hydrocarbon content for the recycleacid, means connected to the sensing means and to the predeterminedhydrocarbon content signalmeans for providing a control signalcorresponding to a desired interface level between the acid phase andthe hydrocarbon phase in, the acid settler, and means connected to thecontrol signal means for controlling the interface level in the acidsettler in accordance with the control signal so as to control thehydrocarbon content of the recycle acid.

2. A system for maintaining a predetermined hydrocarbon content forrecycle acid in analkylation unit and said alkylation unit'includes acontactor wherein an olefin-isoparaffin mixture is contacted withacidand the contactor provides an acid-hydrocarbon mixture to a settlerwhich separates a hydrocarbon phase and an acid phase to providehydrocarbon enriched acid and crude alkylate, fresh acid entering thealkylation unit is added to the hydrocarbon enriched acid, a portion ofthe hydrocarbon enriched acid is discharged while a portion of thehydrocarbon enriched acid isrecycled back to the contactor along withthe fresh acid as the recycle acid, comprising means for sensingconditions of the fresh acid. the crude alkylate andthe recycle acid,means for sensing an interface level between the acid phase and thehydrocarbon phase in the settler, means for providing a signalcorresponding to a predetermined target hydrocarbon content for therecycle acid. means for controlling the interface level in the settlerin accordance with a control signal. and means connected to the sensingmeans. to the target hydrocarbon content signal means and to the controlmeans for providing the control signal to the control means inaccordancewith the condition signals. the sensed level 1 signal and thetarget hydrocarbon content signal to control the hydrocarbon content-ofthe recycle acid.

3. A system as described in claim 2 in which the condition sensing meanssenses the densities of the fresh acid, the crude alkylate and therecycle acid and pro- H 10mm in/D4 D) where H is the hydrocarbon contentof the recycle acid and D D and D are the densities of the fresh AL e(H]; HT)

where e is a constant and H is the target hydrocarbon content; and meansconnected to the change signal means, to the level sensing means and tothecontrol means for summing the change signal and the sensed levelsignal to provide a signal corresponding to a desired interface level inthe settler as the control signal.

5. A system as'described in claim 4 further comprising means forproviding reference signals corresponding to upper AL and lower ALlimits on the amount of change in the interface level in the settler andupper L and lower L limits on the interface level in the settler; thechange signal means further comprises first switching means connected tothe summing means and receiving the change AL signal and the AL and ALreference signals for providing the AL signal as the change signal tothe summing means when the change AL is less than the upper AL limit butgreater than the lower AL limit, providing the AL reference signal asthe change signal to the summing means when the change AL is not lessthan the upper AL limit, and providing the AL reference signal as thechange signal to the summing means when the change AL is not greaterthan the lower AL limit; and the summing means further comprises secondswitching means connected to the control means and receiving the desiredlevel signal from the summing means and the L and L reference signalsfrom the reference signal means for providing the desired level signalto the control means at the con-- trol signal when the desired level isless than the upper level limit L and greater than the lower level limitL,,, providing the L reference signal to the control means as thecontrolsignal when the desired level signal is not less than the upper levellimit L and providing the L reference signal to the control means as thecontrol signal when the desired level signal is not greater than thelower level limit L 6. A method for controlling an alkylation unit tocontrol the hydrocarbon content of recycle acid in the al kylation unitand said alkylation unit includes a contactor wherein anolefin-isoparaffin mixture is contacted with acid and the contactorprovides an acidhydrocarbon mixture to an acid settler which separates ahydrocarbon phase and an acid phase to provide hydrocarbon enriched acidand crude alkylate, a portion of the hydrocarbon enriched acid isdischarged while a portion of the hydrocarbon enriched acid is recycledback to the contactor as the recycle acid along with fresh acid enteringthe alkylation unit, which comprises the following steps of sensingconditions of the fresh acid, the crude alkylate and the recycle acid,sensing an interface level between the acid phase and the hydrocarbonphase in the acid settler, providing a signal corresponding to apredetermined hydrocarbon content for the recycle acid. controlling theinterface level in the acid settler in accordance with a control signal,providing the control signal in accordance with the condition signals.the level signal and the target hydrocarbon contents signal for acidcontrol the hydrocarbon con- H 100(D DB)/(D,1 u)

where H, is the hydrocarbon content of the recycle acid and D D and Dare the densities of the fresh acid, the recycle acid and the crudealkylate; providing a signal corresponding to a change AL in theinterface level in the acid settler in accordance with the hydrocarboncontent signal and the target signal and the following equation:

AL e(H,, H

where e is a constant and H is the target hydrocarbon content; andsumming the change signal and the sensed interface level signal toprovide a signal corresponding to a desired interface level in the acidsettler as the cong 10 trol signal.

9. A method as described in claim 8 which further comprises thefollowing steps, providing reference signals corresponding to upper ALand lower AL limits on the amount of interface level change and upper Land lower L limits on the interface level in the acid settler; and thechange signal step further comprises providing the AL signal as thechange signal when the change AL is less than the upper AL limit butgreater than the lower AL limit, providing the AL reference signal asthe change signal when the change AL is not less than the upper AL limitand providing the AL reference signal as the change signal when thechange AL is not greater than the lower AL limit; and the summing stepfurther comprises comparing the sum signal with the L and L referencesignals, providing the sum signal as the control signal when the sumsignal is less than the upper level limit L and greater than the lowerlevel limit L providing the L reference signal as the control signalwhen the sum signal is not less than the upper level limit L andproviding the L reference signal as the control signal when the sumsignal is not greater than the lower level limit L

2. A system for maintaining a predetermined hydrocarbon content forrecycle acid in an alkylation unit and said alkylation unit includes acontactor wherein an olefin-isoparaffin mixture is contacted with acidand the contactor provides an acid-hydrocarbon mixture to a settlerwhich separates a hydrocarbon phase and an acid phase to providehydrocarbon enriched acid and crude alkylate, fresh acid entering thealkylation unit is added to the hydrocarbon enriched acid, a portion ofthe hydrocarbon enriched acid is discharged while a portion of thehydrocarbon enriched acid is recycled back to the contactor along withthe fresh acid as the recycle acid, comprising means for sensingconditions of the fresh acid, the crude alkylate and the recycle acid,means for sensing an interface level between the acid phase and thehydrocarbon phase in the settler, means for providing a signalcorresponding to a predetermined target hydrocarbon content for therecycle acid, means for controlling the interface level in the settlerin accordance with a control signal, and means connected to the sensingmeans, to the target hydrocarbon content signal means and to the controlmeans for providing the control signal to the control means inaccordance with the condition signals, the sensed level signal and thetarget hydrocarbon content signal to control the hydrocarbon content ofthe recycle acid.
 3. A system as described in claim 2 in which thecondition sensing means senses the densities of the fresh acid, thecrude alkylate and the recycle acid and provides corresponding signals.4. A system as described in claim 3 in which the control signal meansincludes means connected to density sensing means for providing a signalcorresponding to the hydrocarbon content of the recycle acid inaccordance with the density signals and the following equation: HB100(DA - DB)/DA - DH) where HB is the hydrocarbon content of the recycleacid and DA, DB and DH are the densities of the fresh acid, the recycleacid and the crude alkylate; means connected to the hydrocarbon contentsignal means and to the target signal means for providing a signalcorresponding to a change Delta L in the interface level in the settlerin accordance with the hydrocarbon content signal and the target signaland the following equation: Delta L e(HB - HT) where e is a constant andHT is the target hydrocarbon content; and means connected to the changesignal means, to the level sensing means and to the control means forsumming the change signal and the sensed level signal to provide asignal corresponding to a desired interface level in the settler as thecontrol signal.
 5. A system as described in claim 4 further comprisingmeans for providing reference signals corresponding to upper Delta LUand lower Delta LL limits on the amount of change in the interface levelin the settler and upper LU and lower LL limits on the interface levelin the settler; the change signal means further comprises firstswitching means connected to the summing means and receiving the changeDelta L signal and the Delta LU and Delta LL reference signals forproviding the Delta L signal as the change signal to the summing meanswhen the change Delta L is less than the upper Delta LU limit butgreater than the lower Delta LL limit, providing the Delta LU reFerencesignal as the change signal to the summing means when the change Delta Lis not less than the upper Delta LU limit, and providing the Delta LLreference signal as the change signal to the summing means when thechange Delta L is not greater than the lower Delta LL limit; and thesumming means further comprises second switching means connected to thecontrol means and receiving the desired level signal from the summingmeans and the LU and LL reference signals from the reference signalmeans for providing the desired level signal to the control means at thecontrol signal when the desired level is less than the upper level limitLU and greater than the lower level limit LL, providing the LU referencesignal to the control means as the control signal when the desired levelsignal is not less than the upper level limit LU and providing the LLreference signal to the control means as the control signal when thedesired level signal is not greater than the lower level limit LL.
 6. Amethod for controlling an alkylation unit to control the hydrocarboncontent of recycle acid in the alkylation unit and said alkylation unitincludes a contactor wherein an olefin-isoparaffin mixture is contactedwith acid and the contactor provides an acid-hydrocarbon mixture to anacid settler which separates a hydrocarbon phase and an acid phase toprovide hydrocarbon enriched acid and crude alkylate, a portion of thehydrocarbon enriched acid is discharged while a portion of thehydrocarbon enriched acid is recycled back to the contactor as therecycle acid along with fresh acid entering the alkylation unit, whichcomprises the following steps of sensing conditions of the fresh acid,the crude alkylate and the recycle acid, sensing an interface levelbetween the acid phase and the hydrocarbon phase in the acid settler,providing a signal corresponding to a predetermined hydrocarbon contentfor the recycle acid, controlling the interface level in the acidsettler in accordance with a control signal, providing the controlsignal in accordance with the condition signals, the level signal andthe target hydrocarbon contents signal for acid control the hydrocarboncontent of the recycle acid.
 7. A method as described in claim 6 inwhich the sensed conditions are the densities of the fresh acid, thecrude alkylate and the recycle acid.
 8. A method as described in claim 7in which the control signals step include providing a signalcorresponding to the hydrocarbon content of the recycled acid inaccordance with the density signals and the following equation: HB100(DA - DB)/(DA - DH) where HB is the hydrocarbon content of therecycle acid and DA, DB and DH are the densities of the fresh acid, therecycle acid and the crude alkylate; providing a signal corresponding toa change Delta L in the interface level in the acid settler inaccordance with the hydrocarbon content signal and the target signal andthe following equation: Delta L e(Hb - HT) where e is a constant and HTis the target hydrocarbon content; and summing the change signal and thesensed interface level signal to provide a signal corresponding to adesired interface level in the acid settler as the control signal.
 9. Amethod as described in claim 8 which further comprises the followingsteps, providing reference signals corresponding to upper Delta LU andlower Delta LL limits on the amount of interface level change and upperLU and lower LL limits on the interface level in the acid settler; andthe change signal step further comprises providing the Delta L signal asthe change signal when the change Delta L is less than the upper DeltaLU limit but greater than the lower Delta LL limit, providing the DeltaLU reference Signal as the change signal when the change Delta L is notless than the upper Delta LU limit and providing the Delta LL referencesignal as the change signal when the change Delta L is not greater thanthe lower Delta LL limit; and the summing step further comprisescomparing the sum signal with the LU and LL reference signals, providingthe sum signal as the control signal when the sum signal is less thanthe upper level limit LU and greater than the lower level limit LL,providing the LU reference signal as the control signal when the sumsignal is not less than the upper level limit LU and providing the LLreference signal as the control signal when the sum signal is notgreater than the lower level limit LL.